In the petroleum industry, wells for the production of fluids from subterranean hydrocarbon bearing formations are often completed in formations which are partially or even completely unconsolidated, thus resulting in the flow of particulate materials such as sand grains into the well where they accumulate. In other cases, the productive formation may be characterized by good cementation, but unwanted particulate materials may accumulate in the well as a result of treatment procedures which are carried out to increase the gross permeability or flow capacity of the formations.
Conventional well treatment procedures include hydraulic fracturing and acidizing. Hydraulic fracturing involves the injection of a hydraulic fracturing fluid into the well, and the imposition of sufficient pressure on the fracturing fluid to cause the formation to mechanically break down with the attendant formation of one or more fractures. The fractures formed may be horizontal or vertical with the later usually predominating and with the tendency toward vertical fracturing orientation increasing with the depth of the formation being treated. Simultaneously with or subsequently to the formation of a fracture at least a portion of the fracturing fluid comprising a thickened carrier fluid having a propping agent such as sand or other particulate material entrained therein is introduced into the fracture. The propping agent is deposited in the fracture and functions to hold the fracture open after the pressure is released and the fracturing fluid produced back into the well.
Another effective procedure for increasing the gross or apparent permeability of a subterranean hydrocarbon bearing formation is acidizing. In acidizing, an aqueous solution of a suitable acid is injected into the well and forced into the surrounding formation where it dissolves acid soluble material therein to form relatively small fissures or fractures. Acidizing procedures are usually applied to carbonate containing formations and suitable acids for use in such formations include hydrochloric, formic and acidic acids. In some cases, however, sandstones containing little or no carbonate materials may be treated with acids such as hydrochloric or hydrofluoric acids or blends thereof.
Acidizing and mechanical fracturing also may be applied in a common procedure in which an acidizing fluid, usually in the form of a relatively low viscosity "spearhead," is injected into the well under sufficient pressure to break down the formation and produce fractures by hydraulic fracturing. The spearhead fluid may be followed by a higher-viscosity fluid containing a propping agent, which may be an acidic or a conventional non-acidic fracturing fluid.
In such fracturing processes, it is sometimes expedient to employ a fluid loss additive in all or part of the fracturing fluid. In hydraulic fracturing, the fluid loss additive functions to minimize loss of fracturing fluid into the formation as the formation breakdown pressure is reached, thus aiding in initiation of the fracture. Also, as the fracture is formed, fracture propagation outwardly into the formation is enhanced since the fluid loss additive functions to decrease filtrate loss through the walls of the fracture into the formation matrix.
Treating or stimulating procedures such as those described above often times result in an accumulation of unwanted particulate material in the bottom of the well. For example, some propping sand may settle out of the fracturing fluid as it is forced from the well into the formation. Lost circulation material may likewise sometimes accumulate in the bottom of the well. Also, at the conclusion of the fracturing procedure, a substantial quantity of propping sand is produced back from the formation into the well where it accumulates. The use of acidizing fluids may also result in the accumulation of unwanted materials within the well. For example, an acidizing fluid may react with various metallic materials to produce precipitates or gel-like flocculants which gather in the well.
The flow of unwanted particulate materials into a well and/or the accumulation of such detrimental materials therein can present a number of problems. In the case of gas wells, sand material may flow into the well through perforations or liner slots in the form of high velocity jets which can lead to errosion of downhole well equipment. Often times gas wells are completed in a manner in which a single production interval of the well is open to a plurality of gas sands, permitting for co-mingled production from the several sands through a single tubing string. Detrimental material flowing into the well tends to accumulate in the bottom of the production interval, thus restricting production from the lower sands. This problem can be particularly pronounced when the well is placed on production after stimulation with a procedure such as acidizing or hydraulic fracturing. Especially in the case where an accumulated sand column contains produced liquids or liquids used in stimulation, the flow of fluid from the formation into the bottom of the well can be all but stopped.
Similar difficulties may be encountered where only one producing horizon is involved. Here, the problem can be exacerbated by the fact that the closing off of perforations in the lower portion of the producing zone will cause the gas entering the well from the remaining open perforations to be at even a greater velocity than would otherwise be the case, thus further causing errosion of any downhole well equipment which may be subject to the blast zone conditions.
While serious sanding problems are most often encountered in conjunction with gas productions, they may also occur in the case of oil production. In this case, sand entrained in the oil can cause damage to downhole equipment such as the standing and traveling valve units of a sucker rod pumping unit. Sand can also actually accumulate about the pump, or the gas anchor, if any, associated with the pump, restricting the flow of fluids into the pump barrel.
Various method have been proposed for the removal of accumulated detrital material from a well. For example, as disclosed in Uren, L. C. Petroleum Production Engineering - Oil Field Exploitation, "Methods of Removing Detrital Accumulations within the Oil String," McGraw-Hill, Third Edition, 1953, pp. 405-409, a bailer may be lowered into the well to mechanically lift sand from the well. Another procedure involves lowering the tubing string until it is just above the column of accumulated detrital material and then circulating oil down through the tubing with a return of oil and entrained sand through the tubing-casing annulus. As the detrital material is removed, the tubing is gradually lowered until the bottom is reached. Another procedure involves circulation of compressed air or gas down through the tubing together with a small amount of water and oil. The tubing is lowered into the accumulated detrital material which is returned to the surface through the tubing-casing annulus by the action of the rapidly expanding gas as it flows upwardly through the annulus.
U.S. Pat. No. 3,572,431 to Hammon discloses an apparatus for retrieving downhole material such as various pieces of junk, debris and the like or accumulated mud and sand. In Hammon, the retrieval apparatus is attached to the lower end of a pipe string and introduced into the bottom of the well adjacent the accumulated debris, sand or mud. The Hammon apparatus comprises a hollow cylindrical body which includes a cylindrical basket of reduced dimension to define a space between the exterior of the basket and the internal cylindrical body. A catcher assembly, including pivoted flaps, is located near the bottom of the basket, immediately above a plurality of teeth formed at the extreme lower end of the external cylindrical member. Fluid is circulated down the annulus surrounding the drill pipe and passes up through the lower opening and catcher assembly into the interior of the basket and then into the interior passage of the pipe. Accumulated debris is held in the basket by the catcher assembly. After the basket is filled, circulation can still be maintained through the basket annulus in order to clean out sand, mud and the like at the bottom of the well.
U.S. Pat. No. 4,211,280 to Yeates discloses a completion tool which involves a tubular nipple unit including an optional catcher sub having side production apertures and a hydraulic pressure relief port at the bottom. The unit is run into the well at the lower end of a tubing string with an ejectable surge plug in place above the production apertures. A drop bar is employed to eject the surge plug from the nipple into the optional catcher sub. Ejection of the surge plug causes a rapid pressure differential causing fluid and debris within the well bore to surge upwardly within the tubular member.